1. Field
The present embodiments relate to a liquid crystal display (LCD) device and a method of fabricating the same.
2. Related Art
A display device can be used as a visual information transfer medium. It is desired that a display device has lower power consumption, a thin thickness, a light weight and high image quality. An LCD device, which is a flat panel display (FPD) device, satisfies the aforementioned characteristics and is easily mass-produced. A variety of new products using the LCD device have been developed. The LCD device has become a core component which has gradually replaced a general cathode ray tube (CRT).
In general, the LCD device can display images by controlling optical transmissivity of liquid crystal cells aligned in a matrix shape. The LCD device individually supplies data signals to the liquid crystal cells according to image information.
An active matrix (AM) method, which is a driving method commonly used in the LCD device, drives liquid crystals of a pixel unit using amorphous silicon thin film transistors (a-Si TFT) as switching elements. Accordingly, a low-priced insulating substrate can be employed because a low temperature process can be performed on the a-Si TFT. The a-Si TFT has been used more frequently.
However, electric mobility (˜1 cm2/Vsec) of the a-Si TFT is not desired for a peripheral circuit requiring a high speed operation over 1 MHz. Therefore, researches have integrated a pixel unit and a driving circuit unit on a glass substrate at the same time by using a polycrystalline silicon (poly-Si) TFT having higher field effect mobility than the a-Si TFT. Accordingly, a driving circuit can be installed directly on a substrate because the poly-Si TFT has low photographic sensitivity and high field effect mobility.
By increasing the mobility, an operating frequency of the driving circuit unit that determines a number of driving pixels is improved. Accordingly, the display device has high minuteness. A charging time of a signal voltage in the pixel unit is reduced to prevent distortion of transferred signals. Thus, quality of the image is improved.
FIG. 1 is a schematic plane view illustrating the structure of a related art LCD device. FIG. 1 illustrates a driving circuit-integrated LCD device which integrates a driving circuit unit on an array substrate. The LCD device includes a color filter substrate 5, an array substrate 10, and a liquid crystal layer (not shown) formed between the color filter substrate 5 and the array substrate 10.
The array substrate 10 includes a pixel unit 35 which is an image display region in which unit pixels are arranged in a matrix shape. A driving circuit unit 30 is positioned outside the pixel unit 35 and includes a data driving circuit unit 31 and a gate driving circuit unit 32.
Although not illustrated, the pixel unit 35 of the array substrate 10 includes a plurality of gate lines and data lines arranged on the array substrate 10 in the horizontal and vertical directions that define a plurality of pixel regions, TFTs that are switching elements formed in the crossing regions of the gate lines and the data lines, and pixel electrodes formed in the pixel regions.
The TFTs are switching elements that apply a signal voltage to the pixel electrodes and intercept the signal voltage. The TFTs are field effect transistors (FET) that control current flows by an electric field.
The driving circuit unit 30 of the array substrate 10 is positioned outside the pixel unit 35, for example, in the region of the array substrate 10 that extends outside the color filter substrate 5. The data driving circuit unit 31 is positioned at one side of the protruded array substrate 10, for example, the long side of the protruded array substrate as shown in FIG. 1. The gate driving circuit unit 32 is positioned at another side of the protruded array substrate 10, for example, the short side of the protruded array substrate 10 as shown in FIG. 1.
The data driving circuit unit 31 and gate driving circuit unit 32 include CMOS structured TFTs, which are inverters that output the input signals. For example, the CMOS is a kind of integrated circuit having a MOS structure which is used in the driving circuit unit TFT requiring high speed signal processing. The CMOS needs an n channel TFT and a p channel TFT and has an intermediate speed and density between NMOS and PMOS.
The gate driving circuit unit 32 and the data driving circuit unit 31 supply scan signals and data signals to the pixel electrodes through the gate lines and the data lines, respectively. The gate driving circuit unit 32 and the data driving circuit unit 31 are connected to an external signal input terminal (not shown) to control external signals from the external signal input terminal and output the resultant signals to the pixel electrodes.
Color filters (not shown) that embody colors and common electrodes (not shown) that face the pixel electrodes formed on the array substrate 10 are formed in the pixel unit 35 of the color filter substrate 5.
A cell gap is maintained by spacers (not shown) so that the color filter substrate 5 and the array substrate 10 can be isolated from each other. The color filter substrate 5 and the array substrate 10 are attached to each other by seal patterns (not shown) formed at the outer portions of the pixel unit 35, thereby forming a unit LCD panel. The color filter substrate 5 and the array substrate 10 are attached to each other through absorption keys.
The driving circuit-integrated LCD device using the poly-Si TFTs has excellent device properties, for example, high image quality, small size and low power consumption.
In the driving circuit-integrated LCD device, the pixel unit TFTs and the circuit unit TFTs must be formed on the same substrate. Generally, both the n channel TFTs and the p channel TFTs must be formed in the circuit unit. As compared with the a-Si TFT LCD device forming single type channels, the driving circuit-integrated LCD device complicates the production process.
A photolithography process is performed to fabricate the array substrate including the TFTs. The photolithography process forms target patterns by transferring patterns formed on a mask to a thin film-deposited substrate. The photolithography process includes a plurality of processes, for example, coating of photosensitive solution, exposure and development. As a result, the plurality of photolithography processes decrease the production yield and increase a probability of generating defects on the TFTs.
The mask that forms the patterns is very expensive. If the number of the masks increases, the production cost of the LCD device increases.